The woodchuck hepatitis virus (WHV) is taxonomically and serologically related to the hepatitis B virus (HBV). Infection with each of these viruses is associated with acute and chronic hepatitis and hepatic cell carcinoma in their respective hosts; these associations appear to be etiological in nature. The WHV/woodchuck model system provides a convenient means of studying the relationship between virus and host in the oncogenic process. Tests specific for the WHV antigen-antibody systems have been developed. Genetic relationships between the hepadnaviruses and retroviruses suggest a common origin. Genetically altered WHV genomes, constructed by recombinant DNA techniques are being studied in woodchucks in order to determine the biological significance of individual gene products.